This is a list of possible replacement stems that fit a headshok:
1) Roox Danny HS stem (this is the one I ended up getting)--available on eBay or at http://www.monkamoo.com/catalog/home.html
2) Profile Stiffy OS
3) Woodman Wow HS (www.woodmancomponents.com)
4) Woodman Wow HS4
5) Kore Elite
6) Cannondale offers replacement stems.
Also, you can use any mountainbike handlebar with these stems. I have a monkelite on my F600 and love it.